Perform a switch riding while jibbing.

Achievement Guide for Jib & Switch

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find any rail in the game (I like the two long straights in the balloon busters level 1 right up front in the middle)

you want to jump on any rail and step (don't jump!) left foot in front of right foot and back again as many times as you can manage (prob 3 or 4 switches) while on the rail, try and keep your feet close to the floor don't lift your knees to much.